Correlation of structure changes in the water-induced phase transitions of Poly(ethylenimine) viewed from molecular, crystal, and higher-order levels as studied by simultaneous WAXD/SAXS/Raman measurements

Simultaneous measurements of WAXD, SAXS, and Raman spectra have been performed during the water-induced phase transitions of poly(ethylenimine) (PEI). As is well-known, PEI transforms among four kinds of crystalline forms by absorbing water: anhydrate (El/water = 1/0), hemihydrate (1/0.5), sesquihydrate (1/1.5), and dihydrate (1/2). The chain conformation changes from double helix in the anhydrate (0) to planar-zigzag form in the hemi- (0.5), sesqui- (1.5), and dihydrates (2). The structuctural changes in the crystalline region during the water-induced phase transitions have been found to induce also the remarkable changes in the higher-order structure of stacked lamellae on the basis of the detailed comparison between WAXD and SAXS data. The anhydrate (0) composed of double helices showed the long period of 81 Å. When the anhydrate (0) started to transfer to the hemihydrate (0.5), the two types of long periods, 75 and 159 Å, were observed. The 159 Å period appeared only temporarily in the transition process from anhydrate (0) to hemihydrate (0.5). In the transition from hemihydrate (0.5) to sesquihydrate (1.5) and to dihydrate (2) the long period changed to 64 Å (0.5 and 1.5) and 61 Å (2). These SAXS profile changes have been reproduced quantitatively on the basis of a stacked lamellar structural model with paracrystalline disorder taken into consideration.